The present invention relates to a test standard for a coordinate-measuring instrument, in the form of a ball plate which contains a plurality of balls lying in the same plane.
In the testing of a coordinate-measuring instrument, current practice generally uses gauge blocks of graduated standardized lengths, which are set up in different orientations, for example, parallel or diagonal to the measurement axes of the coordinate-measuring instrument and within the measurement volume thereof. These tests are very time-consuming; they must not only be performed at the factory as part of final inspection of the completed coordinate-measuring instrument, but these tests must also be performed at regular intervals at the user's location. Furthermore, testing by means of standardized gauge blocks can only yield single-dimensional predictions as to the errors of the measuring instrument. Since the entire course of such testing requires a very long period of time, there is the further danger that temperature variation can impair the precision of measurement.
It has already been proposed to test coordinate-measuring instruments by means of ball plates. Thus, for example, U.S. Pat. No. 4,364,182 describes a ball plate having a three-dimensional distribution of balls. Such test bodies, however, do not have sufficient inherent stability, so that the distances between the balls can change relatively easily. Frequent recalibration of the test piece itself is therefore necessary.
The journal "Technisches Messen", Volume 51, 1984, Issue No. 3, at pages 83-95, describes various calibration-test bodies, including bodies in the form of a flat perforated plate and in the form of a flat ball plate. In the latter case, the balls are so positioned by means of cylindrical stilts on the surface of the flat support body that all balls lie in the same plane. Such a calibrated test device is then set up in several positions, for example, in a skewed orientation, within the measurement volume of the coordinate-measuring instrument to be tested.
This known test device has only limited longtime stability. It is also relatively heavy and has the disadvantage that the balls which it mounts can only be contacted from one side.
The aforementioned test bodies, such as gauge blocks, ball plates and perforated plates, must furthermore be calibrated. This is possible only at calibration stations equipped for this purpose, such as, for example, the Federal Physical-Technical Institute (Physikalisch-Technischen Bundesanstalt), or the National Bureau of Standards. Recalibration at regular time intervals therefore means a relatively great expenditure of time and money.